The retromer is an endosome-associated sorting complex that selects and packages integral membrane proteins for transport from the endosome to the trans-Golgi network (TGN) or via the TGN to the plasma membrane. Because sorting by the retromer determines whether endocytosed plasma membrane proteins are recycled back to the plasma membrane or routed to lysosomes for degradation, the retromer plays a key role in regulating the composition of the plasma membrane and hence in the cellular response to external stimuli. The retromer is also important for the correct trafficking of lysosomal hydrolases and hence for lysosome biogenesis and homeostasis, and its function is exploited in the replication of a number of human pathogens. Consistent with a fundamental role in membrane trafficking, retromer dysfunction has been linked to human diseases, including Alzheimer's as well as spastic paraplegia. Nevertheless, how this important complex carries out a major function, cargo selection, is poorly understood. To better understand the selection process, we will use structural techniques to visualize the interaction of the retromer cargo selection complex (CSC), the subcomplex responsible for cargo recognition, or appropriate CSC subassemblies with peptides bearing cargo recognition sequences. These structures will allow us to delineate molecular rules by which retromer recognizes its cargo, guiding the identification of new molecules that require retromer for their trafficking. Our studies should also reveal conformational changes in the CSC that take place upon cargo binding, giving first insights as to how cargo selection may be coordinated with cargo packaging, the other major retromer function. Purified protein complexes for structural studies are in hand, and initial crystallization conditions have been identified.